1. Field of the Invention
This invention relates to a helicopter rotor having an even number of blades wherein the opposed blades have a common rotor hub attachment member in the form of a flexible spar, or flexbeam. Generally rectangular in cross-section, the spar reacts the centrifugal load between opposite blades and may extend the full rotor diameter from blade tip to tip or may terminate at a convenient radial station. The spar is designed for extreme flexibility in torsion and flapwise bending to accommodate pitch change and blade flapping excursions respectively, while exhibiting a lesser degree of flexibility in the in-plane direction where lead-lag motions are encountered. This invention is adaptable to rotor configurations wherein the blade to hub attachment means consists of upper and lower plate members which rigidly clamp together around the blade spar. Pitch change is introduced to the blade through conventional control rods to a horn on the inner end of a torque tube which connects to the blade and spar at a radial distance sufficient for the spar to tolerate twisting between its inboard clamped station and its outboard blade connection station. The torsional stress reacted by the spar produced by pitch change twisting is a function of this length, and is proportional to the slope of the curve resulting from a plot of spar angle of attack to radial length. The particular area in which this invention is most concerned is the possible adverse coupling effect produced when one deliberately introduced motion imposed on a blade tends to introduce a motion of another type; in some cases an adverse or unstabilizing motion. This tendency to produce adverse coupling effects is most frequently exhibited when the three degrees of rotor blade freedom (pitch, flap, and hunting, i.e., lead-lag) are reacted by a single joint or flexure member rather than by individual articulated joints or hinges. This invention is directed to isolate lead-lag motions of the spar member from pitching motions. Heretofore, a lead-lag (in-plane) motion tended to increase or decrease the pitch horn radius, and thus provide a lesser or greater degree of pitch change than desired.
2. Description of the Prior Art
The most significant phase of development of the cross beam rotor has occurred in the last 10 years, and the efforts of the assignee of this invention is revealed in the paper entitled, "Composite Bearingless Tail Rotor for UTTAS", dated May 1976 by R. Fenaughty and W. Noehren presented at the 32nd Annual National V/STOL Forum of the American Helicopter Society, the contents of which are incorporated herein by reference. The basic concept of the common spar for opposed blades is depicted therein and also in the Arcidiacono Canadian Pat. No. 951,301 wherein a rigid torque tube having a pitch horn at its inboard end is utilized to introduce pitch changes to the airfoil shaped blade connected to the flexible spar and outboard end of the torque tube. With no centering member between the spar and torque tube at the inner end, an adverse pitch-flapwise bending coupling was introduced. This led to the use of a snubber in the form of an elastomeric bearing, centering the spar within the torque tube at the pitch horn station. This development is depicted in the paper and was incorporated in the YUH-60A prototype helicopters produced by Sikorsky Aircraft, commonly identified at UTTAS.
An early known invention is the U.S. Pat. No. 3,484,174 to McCoubrey which allows for blade flapping excursions by a gimbaled hinge. None of these references address the specific problem of pitch/lead-lag coupling nor depict structure which will prevent or reduce it. One reason for this omission is thought to be the general heretofore limitation of the usage of the cross-beam rotor to helicopter tail rotor or windmill applications. In these applications rotor diameters are relatively short and blade masses smaller than the full 40 to 100 feet diameter helicopter main rotors. Lead-lag blade excursions are of less magnitude and pitch coupling efforts are less severe in tail rotors and can be tolerated. As the state of the art advances toward introduction of the cross beam type rotor as a main rotor, it becomes desirable to address the pitch coupling situation, and such is the objective of this invention.
A further reference depicting the prior art rotor is the U.S. Pat. No. 3,874,820 to R. Fenaughty. This patent addresses the angular relationship between the spar and airfoil members and is an embodiment to which the invention presented here is specifically adaptable. Further, this same embodiment is also depicted in currently pending U.S. application Ser. No. 590,773 of E. Hibyan and W. Noehren filed June 26, 1975. Torque tube structure and hub plate attachment members similar to the structure of this invention are shown in detail therein.
Other art in the field of cross beam rotors that is known but not specifically pertinent due to the absence of torque tube attachment members is the U.S. Pat. No. 3,874,815 to Baskin issued April 1, 1975 and the currently pending U.S. patent application Ser. No. 751,006 of R. Rybicki filed Dec. 15, 1976.